ABSTRACT ? PROJECT 3, MEKK3-ERK5-KLF signaling in CCM pathogenesis Cerebral cavernous malformation (CCM) is a common inherited cerebrovascular disease, and a major cause of stroke and neurologic deficit in younger individuals. Genetic studies in humans have shown that CCMs are caused by loss of three adaptor proteins (KRIT1, CCM2 and PDCD10) that function in a common signaling complex in the endothelial cells that line blood vessels. However, how this signaling complex prevents CCM formation is not known and there are presently no medical therapies to treat this neurovascular disease. We have used genetic studies in developing mice and fish to gain new insight into the downstream mechanisms and effectors of CCM signaling in endothelial cells in vivo. Our studies in the developing heart reveal a new and unexpected mechanism by which CCM signaling regulates endothelial gene expression through effects on the MEKK3 MAPK cascade and the KLF transcription factors. Our studies demonstrate this regulatory mechanism to be causal for the developmental defects associated with endothelial loss of the CCM pathway, and similar gene expression changes are seen in CCM-deficient endothelial cells in the brain. This proposal will extend these developmental studies to test whether increased MEKK3-ERK5-KLF signaling also underlies CCM formation in the brain. To accomplish this we will (i) examine MEKK3-ERK5-KLF signaling in human and mouse CCMs, (ii) test whether genetic or pharmacologic manipulation of MEKK3-ERK5-KLF signaling can rescue of CCM formation in mice, and (iii) determine whether changes in this MAPK pathway underlie the many other signaling abnormalities described in CCMs and CCM-deficient endothelial cells. Our studies will complement those of Projects 1 and 2 and enable us to fully understand the genetic basis of CCM pathogenesis and the primary signaling abnormalities that underlie CCM formation, key steps in the development of new therapies for this disease.